Author Owen Sheers at home in Talgarth near Brecon, mid Wales. Photograph: Dimitris Legakis for the Guardian

From a distance, Owen Sheers’s new novel appears to be missing a title; the cover looks bare but for the image of a flight of stairs, black against a backdrop of bilious yellow, and it’s not until you have the book in your hands that you make out the words lacquered over the top. Tip it to the light and “I Saw a Man” gleams into view like heat haze over tarmac. Put the book back on the shelf and it sinks back into the picture, leaving you wondering whether you saw anything at all.

The title is taken from the opening verse of Hughes Mearns’s well-known “Antigonish”, which Sheers quotes at the beginning of his novel: “Yesterday, upon the stair,” it goes, “I met a man who wasn’t there./ He wasn’t there again today/ I wish, I wish he’d go away…” It’s a queer little poem, shifting back and forth between witty epigram and soured, creepy nursery rhyme, and the cover realises it beautifully. But it’s also a neat metaphor for the provisional, ambiguous story Sheers has written.

The curtain lifts on the borders of Hampstead Heath on a torpid summer afternoon, at the moment when a man, believing his neighbours’ house to be empty, steps inside. Michael Turner has drifted to London following the sudden death of his wife; in the blank aftermath of the tragedy, he finds himself caught in an oddly accelerated friendship with the family next door. Through the details of their connected lives, interspersed with snapshots of Michael’s inching progress through the house, the novel establishes a network of cause and effect that reaches all the way around the world, from the sub-prime mortgage crisis in the US, which sees a father lose his job in London, to the drone operator in a bunker in Nevada who obliterates a man feeding chickens in Pakistan. It twists and turns and plays its cards close to its chest, showing its full hand only in the final pages, when we are forced to reassess everything that has gone before.

The ambiguity that makes Sheers’s novel so compelling is there in him, too: as a writer, he’s impossible to pin down. He started life as a poet, publishing his first collection fresh out of university, but despite racking up great reviews, a clutch of prize shortlistings and a tour with Carol Ann Duffy, Sheers decided that his next move would be to up sticks for Zimbabwe to research the life of his great-great-uncle, Arthur Cripps. The book that arose from his story, The Dust Diaries, blended fact, fiction and conjecture to the extent that it’s sold as non-fiction in some countries and a novel in others; either way, it snagged him the first of many awards (Wales Book of the Year 2005) and the praise of no less an authority than Doris Lessing. From there, he doubled back to publish a second poetry collection, Skirrid Hill, followed that up with a one-man play on the life of the poet Keith Douglas, then brought out his debut novel, Resistance – a crowd-pleasing historical epic that sold more than 50,000 copies. The book before I Saw a Man was Pink Mist, a verse-drama on the Afghan conflict, a stage version of which will have its world premiere at the Bristol Old Vic this July. When we meet he has the first draft of a new play “still warm off the printer. Like a fresh kill” in his bag. And this is just the whistle-stop tour: other highlights include an anthology of British landscape poetry (and a TV series to accompany it), a three-day Passion play co-created with Michael Sheen and a stint as the Welsh Rugby Union’s writer in residence. Truly, he’s contemporary literature’s renaissance man.

Michael Sheen in day three of the Port Talbot Passion. Photograph: Tim Ireland/PA

With a CV like that, his “natural opposition to categorisation” is unsurprising, but when pressed he concedes that in the past he’s called himself “a poet who writes in other forms”. It was only with I Saw a Man that he consciously sought to change the label; here, for the first time, he “wanted, in quite a geeky way, to feel like a novelist writing a novel”. The fictional elements of The Dust Diaries had given him the taste for invention, but his first novel, which tells the counterfactual story of Wales under Nazi occupation, didn’t ultimately allow him its full licence. “Resistance was based on a piece of history that didn’t happen,” he says, “but to portray it with credibility, every room needed to be properly furnished. This time, I wanted to invent something from scratch.”

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I Saw a Man arose entirely from that single striking image of a man easing open his neighbours’ back door, which dropped into Sheers’ head fully formed, “the first time that’s happened to me. Of course it led to a load of questions: who is he, why is he entering like that, where’s he at in his life?” In the end, he borrowed from his own backstory to fill in the blanks: the setting is based on a real street in Hampstead, where Sheers “rented a couple of flats at a time when, like Michael, I was pretty nomadic”, and the book, which he wrote between other projects over seven years, teases out some of the questions he was grappling with in his own life. “When I started it,” he says, “the idea of family felt very distant; I couldn’t work out how people did it.” Three times, he got to 10,000 words before beginning again, “which was a big psychological kick in the teeth; it’s much easier when a poem doesn’t work. But it did mean that by the time I found the right voice, that possibility of the domestic was much closer. My final deadline was my wedding.”

These days, Sheers is a father as well as a husband, and (like Michael and his wife in I Saw a Man) has lately left London for Wales’s Black Mountains. Although he was born in Fiji, where his parents were working for the Ministry of Overseas Development, he did the bulk of his growing up in a small village in Abergavenny, one of just 14 children at the local primary school. Wales – and specifically, that bleak, beautiful corner of it – has been lodestone and muse for him: the Black Mountains’ furthest outlier, Skirrid Hill, provides both title and unifying metaphor for his gorgeously elegiac second poetry collection, and Resistance is set in one of the area’s most remote valleys. Yet though its influence runs through his work, for the duration of his professional life he has lived elsewhere, only recently finding his way home again. Oxford and the University of East Anglia’s creative writing course came first; it took him a long time to find his feet (“There were more kids from Eton in my college than had gone to Oxbridge from my school for a decade, and there seemed to be a secret rule book that everyone else had access to”), and his affinity for his homeland was enhanced through his absence from and longing for it.

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By the time he left UEA, he was “skint. So I applied for [TV production company] Planet 24’s graduate programme and ended up working on The Big Breakfast”. I goggle; he laughs. The contrast with UEA’s rarefied environment was marked; suddenly, he found himself “in a living version of Martin Amis’s Money, inventing puns and booking novelty acts at four in the morning and researching Arthur Cripps in the afternoon. It was a very odd split life.” In the end, a rupture was forced. Arts Council Wales offered him a grant to work on The Dust Diaries, so he quit his job and moved into his parents’ caravan on the Pembrokeshire coast for a winter, where “the day’s main event was walking along the cliffs. The grand idea was that I was going to write and surf, but there weren’t any waves that winter. So I got on with writing.”

Sheers’s site-specific play
Mametz performed by National Theatre Wales in Great Llancayo Upper Wood near Usk in 2014. Photograph: Mark Douet/PR

The Dust Diaries made a minor impact, but it was with Skirrid Hill that Sheers really hit his stride. As well as cementing his reputation as a writer of Wales, it sowed the seed of what would become his other essential subject. The opening poem, “Mametz Wood”, visits the scene of one of the first world war’s bloodiest battles, where “even now the earth stands sentinel, / reaching back into itself for reminders of what happened”. War – its tolls, its terrors, its physical and psychological scars – has haunted his work ever since; it wasn’t, he says, something he set out to write about, but one poet led him to another, one conflict to the next. He followed up his radio play on Douglas with a second on Alun Lewis, the Welsh poet whose centenary is this year, and approached the task from a novelist’s perspective in Resistance. Then he moved on to contemporary conflicts, publishing The Two Worlds of Charlie F, a play “based on the experiences of very recently wounded service personnel” in 2012, and Pink Mist in 2013. “Post-9/11 conflicts have run in exact parallel with my writing life,” he explains, “and you feel as though you want to reflect that, even if it’s through historical prisms. Plus, growing up where I did, I knew kids who entered the army at 15. Britain’s the only EU country that allows you to join the army as a child soldier. It’s scandalous, and I wanted to make people aware of that. I keep saying I want to stop, though. I’m quite warred out.”

He hasn’t quite managed this with I Saw a Man: war and Wales still feature, but their retreat from the foreground marks a new stage in Sheers’s career – and might explain why he gives the impression of being faintly bemused by his latest creation. “Before this, if you’d asked me which two books I’d never write, I’d have said, a book set in Hampstead and a novel about novel writing, which this turned out to be. Both tend to drive me mad. But having lived in temporary places in Hampstead surrounded by that sense of establishment, there was a tension that I found I wanted to explore. And I did end up thinking, what’s the point in writing in a form if you’re not going to interrogate it? It’s a negotiation, obviously; the book also has to be an absorbing read. The novels I really love are both. I hope,” he frowns slightly, “this is, too.”

It is. I Saw a Man is Sheers’s most mature and coherent work to date; taut as a thriller, but resonant with motifs of intimacy and distance, guilt and redemption, and the nature of stories and storytelling. The only shame is that his name doesn’t appear on the cover.

‘Something about the Black Mountains lends itself towards myth and magic, folklore and fiction. What exactly, I’m not sure,’ says Oliver Balch. With author and poet Owen Sheers as his guide, he aims to find out